1. Field of the Invention
This invention relates to a semiconductor device using carriers derived from a semiconductor hetero-structure interface.
2. Description of the Prior Art
Gallium arsenide (GaAs), having a much higher mobility of electrons than silicon, is suitable for producing a fast device. However, production of an MOS (Metal-Oxide-Semiconductor) type of field effect transistor using GaAs, similar to that using silicon was impossible, because of the difficulty in the formation of a good insulator when using GaAs. Recently, it has been found that the employment of a hetero-structure of GaAs and aluminum gallium arsenide (AlGaAs) containing donor impurities can provide a field effect transistor owing to the generation of carriers at the interface. FIG. 1 shows the drawing of band structure of the above-mentioned transistor's working area wherein 13 is an electrode portion, 12 an AlGaAs layer containing an impurity, and 11 a GaAs layer containing substantially no impurity. Fe indicates Fermi level. In FIG. 1, 15 indicates the carriers confined in the triangular potential well. Since the carriers 15 which are provided from the donor impurity (14) in AlGaAs (12) transit through GaAs containing no impurities, they are separately placed from the ionized donor impurity. As a result, the scattering of electrons due to the impurity potential can be greatly reduced and high mobility can be achieved. However, in transistors of such structure with high mobility electrons, the transconductance becomes small because the gate voltage is not effectively applied to the interface due to the larger addition of donor impurity. In order to avoid this effect, it is desirable to use AlGaAs containing no impurities as in the MOS structure. However, in the case of a Schottky type gate, there are gaps between the source and drain electrodes and the channel, which differs from the MOS structure. Accordingly, when no donor impurity is added, no carriers are induced in this gap portion, so that the channel and the source-drain electrodes cannot be connected to work as a transistor.
It is to be noted that the above examples are given in Japanese Journal of Applied Physics Vol. 19, No. 5, May, 1980, pp L225-L227 and MICROWAVES, October, 1980, p. 20.
There is proposed a field effect transistor characterized by introducing donor impurities into the gate electrode-side semiconductor having the wide forbidden band in the gap area between the channel and the source-drain electrodes, such as AlGaAs in the above-mentioned example, and by introducing no impurity in the channel portion directly under the gate electrode. FIG. 2 shows the band diagram in the working area of the above-mentioned field effect transistor, wherein 13 is an electrode portion, 12 an AlGaAs layer, 11 a GaAs layer and F.sub.E Fermi level, similar to FIG. 1. Such structure has the following characteristic features.
(1) Mobility increases because there are no impurities in AlGaAs near the channel which act as scattering centers.
(2) Mutual conductance can be increased by effectively applying a gate voltage to the channel portion because the AlGaAs layer can have the same effect as the insulating layer in the MOS structure.
(3) The channel portion and the source-drain electrodes can be connected to work as a transistor by introducing donor impurities in the gap area between the channel and electrodes.
However, in the case of the above-mentioned transistor, the source-drain electrodes are generally formed by ion implantation. In this case, when the composition ratio of Al in AlGaAs is at least 0.25, the activation efficiency of the introduced impurities is not high, so that the resistance in the electrode portions becomes higher, to exert a harmful influence on speeding up the movement of electrons.